It is known that an intake control valve is located in an intake passage at a location upstream of an intake valve to close the intake passage and opens/closes in synchronization with opening/closing of the intake valve. The intake control valve is opened momentarily in the midst of an intake stroke to fill a great deal of intake air into a cylinder by using an inertia supercharging effect or a pressure fluctuation of the intake air (for example, refer to JP-A-2000-248946). Since the intake control valve can open and close during one intake stroke, such supercharging can start at the same time an accelerator pedal is depressed and is better in responsiveness than turbo charging which starts due to rise of a turbine. Therefore, for example, such supercharging is suitable for eliminating an acceleration delay of a vehicle. In addition, the supercharging can increase an intake air quantity more than a normal aspiration, thereby increasing torque generated by the engine.
In the meantime, there is a case where in the engine control, an air quantity flowing into a cylinder is estimated for each cylinder cycle to set a fuel injection quantity, fuel injection timing, ignition timing and the like. In this case, the estimation of the air quantity is made based upon a detection value of an intake air quantity detected by an air flow meter or a detection value of an intake pressure sensor.
In the case of using the intake control valve as described above, however, the flowing intake air quantity changes for each intake cycle in response to operation timing of the intake control valve. Therefore, a method of using the detection value of the air flow meter or the intake pressure sensor can not adapt for responding to the change of the air quantity for each intake cycle, resulting in being incapable of accurately detecting the air quantity. In other words, in the method of using the detection value of the air flow meter or the intake pressure sensor, only an average air quantity flowing into the cylinder can be estimated, and a fluctuation quantity can not be estimated in the case where the air quantity per intake cycle unit changes. Further, the method of using the detection value of the air flow meter or the intake pressure sensor can not adapt for responding to a change of the air quantity in a case where the intake control valve changes from a non-operating state to an operating state or vice versa.
It should be noted that in a variable valve system capable of setting any opening/closing timing of an intake valve, an estimation of an air quantity is possible, estimating a port pressure at a point the intake valve of each cylinder closes. In the system using the intake control valve as described above, however, a port portion creates a subsonic speed, where the pressure is abruptly changed and a temperature of the intake air changes due thereto. Therefore, it is extremely difficult to estimate the port pressure at a point the intake valve closes, by means of the air flow meter or the like and even if the port pressure is estimated, it inevitably results in that the obtained air quantity is inaccurate.
On the other hand, there is a case where in an engine control, a target air quantity as a target value of an air quantity flowing into the cylinder is determined based upon a parameter representative of an engine operating condition such as an engine rotational speed or an accelerator opening to control the air quantity so that an actual air quantity be equal to the determined target air quantity and also to set a fuel injection quantity, fuel injection timing, ignition timing and the like, based upon the target air quantity. In general, an opening of the intake throttle valve is controlled to make an actual air quantity be equal to a target air quantity.
In a case of adopting the aforementioned intake control valve, opening/closing timing of the intake control valve is controlled, thereby leading to controlling an air quantity. Accordingly, for acquiring a desired air quantity, it is required to open/close the intake control valve at appropriate timing.